The present invention relates generally to magneto-resistive storage devices and, more particularly, the present invention relates to magneto resistive storage devices thatminimize magnetic fields at boundary conditions.
Magnetic Random Access Memory (xe2x80x9cMRAMxe2x80x9d) is a non-volatile memory that is being considered for short-term and long-term data storage. MRAM has lower power consumption than short-term memory such as DRAM, SRAM and Flash memory. MRAM can perform read and write operations much faster (by orders of magnitude) than conventional long-term storage devices such as hard drives. In addition, MRAM is more compact and consumes less power than hard drives. MRAM is also being considered for embedded applications such as extremely fast processors and network appliances.
A typical MRAM device includes an array of memory cells, word lines extending along rows of the memory cells, and bit lines extending along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line.
The memory cells may be based on tunneling magneto-resistive (TMR) devices such as spin dependent tunneling junctions (SDT). A typical SDT junction includes a reference layer, a sense layer and an insulating tunnel barrier sandwiched between the reference and sense layers. The reference layer has a magnetization orientation that is set in a known direction so as not to rotate in the presence of an applied magnetic field in a range of interest. The sense layer has a magnetization that can be oriented in either of two directions; the same direction as the reference layer magnetization or the opposite direction of the reference layer magnetization. If the magnetizations of the reference and sense layers are in the same direction, the orientation of the SDT junction is said to be xe2x80x9cparallel.xe2x80x9d If the magnetizations of the reference and sense layers are in opposite directions, the orientation of the SDT junction is said to be xe2x80x9canti-parallel.xe2x80x9d These two stable orientations, parallel and anti-parallel, may correspond to logic values of xe2x80x9c0xe2x80x9d and xe2x80x9c1.xe2x80x9d
The reference layer described above can be made using a soft magnetic layer that is set dynamically by magnetic field from a current-carrying conductor. Alternatively, the magnetization orientation of the pinned layer may be fixed by an underlying antiferromagnetic (AF) pinning layer. The AF pinning layer provides a large exchange field, which holds the magnetization of the pinned layer in one direction. Underlying the AF layer are usually first and second seed layers. The first seed layer allows the second seed layer to be grown with a  less than 111 greater than  crystal structure orientation. The second seed layer establishes a  less than 111 greater than  crystal structure orientation for the AF pinning layer.
Prior art examples of magneto-resistive devices having AF pinning layers are shown in FIG. 1. FIG. 1 depicts a magnetic tunnel junction 10 consisting of several layers, including multiple ferromagnetic layers. Layer 12 is a non-magnetic conductive layer, typically fabricated from tantalum or copper, or other like materials. On layer 12 is fabricated a magnetic seed layer 14, which may be substituted with a fully patterned ferromagnetic seed layer. An AF pinning layer 16 is then fabricated on layer 14 with a ferromagnetic pinned layer 18 being fabricated on layer 16. The tunneling barrier 20, typically made of a dielectric material such as alumina or silicon dioxide, is fabricated on layer 18. Lastly, a ferromagnetic sense layer 22 is fabricated on the barrier layer 20 to complete the magnetic tunnel junction device 10. Strong, stray magnetic fields are produced at the edges of the ferromagnetic layers 14, 18 and 22. The strong stray magnetic fields assist switching of data films in one direction and oppose switching in the reverse direction. This creates an asymmetry in switching.
Accordingly, what is needed is a structure that reduces or eliminates the stray magnetic fields produced at the edges of the ferromagnetic layers within the magnetic tunnel junction devices of the prior art.
According to the present invention, an electromagnetic device, such as a magnetic memory device, is disclosed that includes means for attenuating, reducing or eliminating stray boundary magneto-resistive offset. The device comprises several layers amongst which is a first layer with stray magnetic fields at its boundaries. The attenuating means comprises a sink layer, magnetically coupled to the first layer, to attenuate the stray magnetic fields of the first layer.
In one embodiment, a magnetic memory device is disclosed that includes means for reducing or eliminating magneto-resistive switching offset. The device comprises a sense layer; a pinned layer; a barrier layer placed between the sense and pinned layers such that each layer is geometrically aligned with the other; a pinning layer placed in adjacent alignment with the pinned layer; and a magnetic sink layer, placed adjacent to the pinning layer, to minimize stray magnetic field effects at the boundaries of the sense, pinned and pinning layers. The magnetic sink layer comprises a magnetic layer having a first portion in adjacent alignment with the pinning layer, the first portion functioning as a pinned layer, and a second unpinned portion, extending beyond the alignment of the other layers and the first portion.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.